Various research and development have been conducted on a wide-angle imaging apparatus to efficiently produce an image of an object in a large area. One example is the development of a wide-angle camera with a fish-eye lens.
JP 10(1998)-54939 A (REFLECTION TYPE VIEWING ANGLE CONVERTING OPTICAL DEVICE AND OPTICAL SYSTEM USING THE DEVICE) proposes a system that produces an image of a wide visual field by variously changing the shapes of three mirror surfaces arranged opposite to each other. JP 2000-4383 A (MULTIDIRECTIONAL IMAGE TAKING-IN DEVICE) proposes an optical system that produces a counter image inside a normal image by using a shaft of a panoramic image block as a lens.
JP 2001-94839 A (WIDE VISUAL FIELD IMAGE PICKUP DEVICE AND WIDE VISUAL FIELD IMAGE PICKUP DISPLAY DEVICE) proposes a device that includes a convex main mirror having a hole in the center, a convex sub-mirror having a hole in the center, and a lens arranged in the hole of the sub-mirror. Rays of light are reflected by the main mirror, further reflected by the sub-mirror, and then are imaged. This image and an image formed by the lens are displayed on the device. WO00/41024 (PANORAMIC IMAGING APPARATUS) proposes a system that provides a substantially flat and stigmatic image plane by using two reflectors such as a convex hyperboloidal mirror and a concave ellipsoidal or spherical mirror, a relay system, and an image sensor.
In recent years, an electrical light receiving element, e.g., a solid-state imaging device has been used often. In this case, optical members such as an optical low-pass filter and a near-infrared cutoff filter are arranged between an imaging lens and the imaging device, and thus a relatively long back focal length is necessary. The optical low-pass filter removes moiré by attenuating a high frequency component not less than a spacial frequency component required for taking pictures. The near-infrared cutoff filter corrects a spectral response of an electrical photoreceptor.
However, a wide-angle imaging optical system using, e.g., a fish-eye lens generally requires many lenses. Therefore, the weight of the optical system is increased, and the apparatus becomes large and expensive. Moreover, the generation of chromatic aberration or the like is a problem, and actually this wide-angle imaging optical system is only used for taking pictures with special techniques.
JP 10(1998)-54939 A discloses the formation of an image in the vicinity of the axial direction, while a means for correcting various aberrations is limited. For a multidirectional image capture device of JP 2000-4383 A, incident light from the object side passes through a peripheral light-transmitting plane and a rotating body before reaching a peripheral light-proof plane. Therefore, a transparent material used for the rotating body is thick, and when the transparent material is resin, it is susceptible to temperature changes and takes longer to be molded. When the transparent material is glass, particularly aspherical glass, considerable cost is involved in polishing the glass.
An image pickup device of JP 2001-94839 A can produce two images simultaneously: an image formed in such a manner that rays of light are reflected by the main mirror, further reflected by the sub-mirror, and then are imaged; and an image formed by the lens that is arranged in the sub-mirror. The optical system includes the convex main mirror, the convex sub-mirror, and the concave lens. Moreover, a master lens should be placed between an imaging device and the sub-mirror. In this case, the F number of the master lens is high, and rays of light entering the master lens are divergent light due to the configuration of the optical system. Consequently, the master lens becomes large and complicated.
A panoramic imaging apparatus of WO00/41024 requires a relay optical system to form an intermediate image in the optical system, and thus the entire length of the optical system is increased, resulting in a larger apparatus.
As described above, when an electrical light receiving element is used, it is necessary not only to ensure a relatively long back focal length so that optical members such as an optical low-pass filter and a near-infrared cutoff filter are provided, but also to correct aberration sufficiently. To achieve this, however, relatively many lenses are required, and an optical design to reduce the number of lenses should be addressed.